The present invention relates to a convergence device for providing precise convergence adjustment with sinusoidal current in an efficient manner.
In color television receivers with a three-gun color kinescope with a shadow mask, color pictures are reproduced in general with three primary colors, that is, red, green and blue colors, and in order to excite color dots or dot trios three electron guns are provided. That is, each beam passes through an aperture opening or hole of the shadow mask to excite its respective color dots on a phosphor screen, without exciting the other two colors.
However, color fringing inevitably results because (1) the red, green and blue electron guns are spaced apart from each other and (2) the radius of curvature of the phosphor dot screen is considerably deviated from an ideal radius of curvature so that one may view clear pictures. In order to correct this color fringing, convergence circuits are in general employed in color television receivers, and they are divided into two types based upon their functions, one type for correcting color fringing caused by the construction of a color television receiver itself and the other type for correcting color fringing due to the production variations of electron guns and shadow masks.
The color fringing due to the construction of a color television receiver itself may be corrected by flowing parabolic current through convergence coils, but the correction of color fringing due to the production and assembly variation requires correction with magnets or the like at the center portion of the phosphor dot screen. That is, correction asymetrical with respect to the center of the phosphor dot screen is required.
Meanwhile more precise convergence adjustment is increasingly demanded for high quality picture reproduction. The convergence circuits for completely eliminating color fringing caused by the two reasons described above are extremely complex in construction. For instance if the tolerance of misconvergence after convergence adjustment is 0.5 to 1.0 mm over the whole phosphor dot screen of a color kinescope from 18 to 20 inches in size, the scope of dynamic convergence parabolic current must be changed in the former and last halves of a cycle independently of each other. Power amplifiers each as Class-B amplifiers, have been used to flow asymmetrical currents through convergence coils.
With such convergence circuits precise convergence adjustments are possible but there arises a problem that the power consumption increases. For instance, with a horizontal dynamic convergence circuit utilizing the Class-B amplifier as the power amplifier, the power consumption increases as much as 40 W.
In color television receivers capable of enlarging (zooming) part of a picture, one horizontal scan is made within 10 microseconds out of the 63.5 microsecond horizontal scanning time in the NTSC system even though the convergence adjustment is the same. As a result, the waveform of the parabolic convergence current must be changed to a considerable degree. As a result, the power consumption further increases with the Class-A amplifier type horizontal convergence circuits.